Japanese Patent Application No. 2001-273881, filed on Sep. 10, 2001, is hereby incorporated by reference in its entirety.
The present invention relates to a light emitting device that uses electroluminescence (EL).
It is expected, that an EL light-emitting element that employs electroluminescence (EL) will be employed in bright, large-capacity displays. Such an EL light-emitting element would be required to have an even greater light emitting efficiency.
In addition, it is desirable to employ EL light-emitting elements that can be fabricated easily as modulation devices in the communications field, but since the light emitting efficiency of EL light-emitting elements is not yet high enough to enable their application to modulation devices in the communications field, it is difficult to apply EL light-emitting elements to modulation devices in the communications field.
The present invention may provide a light emitting device that makes it possible to achieve a further improvement in light emitting efficiency.
A light emitting device in accordance with the present invention comprises a substrate and a light-emitting element section formed on the substrate; the light-emitting element section comprising:
a light-emitting layer capable of generating light by electroluminescence; and
a cathode and anode for applying an electrical field to the light-emitting layer;
wherein the cathode and anode comprise a layer formed of a ferromagnetic material and are also magnetized.
In this case, a ferromagnetic material is a material that is ferromagnetic. Ferromagnetism is a property whereby the positive exchange interaction of magnetic atoms or free atoms of a metal ensure that magnetic moments are aligned in parallel to create spontaneous magnetization.
Note that xe2x80x9cthe cathode and anode comprise a layer formed of a ferromagnetic materialxe2x80x9d means either that the entire cathode or anode is formed of a layer of a ferromagnetic material or that part of the cathode or anode is formed of the ferromagnetic material.
An example of an electrode that is formed in part of a ferromagnetic material is an electrode of a multi-layer film of a ferromagnetic material formed on a surface of a layer formed of a paramagnetic material facing the light-emitting layer.
In this case, xe2x80x9cthe cathode and anode are magnetizedxe2x80x9d means that a magnetic field acts on the ferromagnetic materials of the anode and the cathode to put the direction of magnetic moments of the atoms within that ferromagnetic materials into an aligned state.
Since the light emitting device in accordance with the present invention makes it possible to align the spin orientation of electrons injected from the cathode as well as the spin orientation of holes injected from the anode, by comprising a layer of a ferromagnetic material within the magnetized anode and cathode, it is possible to increase the probability that singlet excitons will be created among all the excitons that are created. This enables an increase in the light emitting efficiency of the fluorescence. For further details, see the sections on the embodiments of the present invention.
A light emitting device in accordance with the present invention can have any of the aspects set out in (1) to (10) below.
(1) The ferromagnetic material could be a half-metal. In this case, a half-metal is one of these ferromagnetic materials which is completely spin-polarized at the Fermi level.
In this case, xe2x80x9cspin-polarized at the Fermi levelxe2x80x9d is a state in which the number of electrons having an up spin state is different from the number of electrons having a down spin state, and xe2x80x9ccompletely spin-polarized at the Fermi levelxe2x80x9d means that either the number of electrons having an up spin state or the number of electrons having a down spin state is zero.
(2) The direction of Fermi-level spin polarization in the ferromagnetic material forming the anode and the direction of Fermi-level spin polarization in the ferromagnetic material forming the cathode could be parallel. In that case, the orientation of spin polarization at the Fermi level means the spin orientation of the majority of the electrons in a state of spin polarization, of the electrons at the Fermi level.
(3) The cathode and the anode could be magnetized in the same direction. In such a case, the direction of magnetization of each electrode (the cathode or anode) is the direction in which the entire film that forms that electrode is magnetized, and is the spin orientation of the majority of electrons within those electrons at a state that is lower than the Fermi level.
Alternatively, the cathode and the anode could be magnetized in opposite directions.
(4) The light-emitting layer, the cathode, and the anode could be formed in a stack on the substrate.
(5) The light-emitting layer, the cathode, and the anode could be disposed perpendicular to the surface direction of the substrate. In such a case, the surface direction of the substrate is a direction parallel to the surface of the substrate in contact with the light-emitting element section.
(6) The cathode and anode could be formed of materials such that the work function of the material forming the cathode is less than the work function of the material forming the anode. In this case, the xe2x80x9cwork functionxe2x80x9d is the energy required for causing electrons at the Fermi level to move to infinity. The above described configuration makes it possible to reduce power consumption, since it enables a reduction in the drive voltage of the light emitting device.
(7) A layer composed of a non-magnetic material could be formed between the cathode and the anode. Such a configuration would make it possible to transport electrons within the insulating substance in a state in which the spin orientation of the electrons is maintained. This makes it possible to maintain the light emitting efficiency.
In such a case, the layer composed of a non-magnetic material could comprise at least the light-emitting layer. This layer composed of a non-magnetic material could also comprise an electron transportation/injection layer and a hole transportation/injection layer.
(8) The light-emitting layer could comprise an organic light-emitting material that generates light by electroluminescence.
(9) The light-emitting layer could comprise a host material and a guest material, where the host material creates excitons and the guest material is excited by the migration of energy from the excitons to generate light.
(10) The device could further comprise at least one of a hole transportation/injection layer and an electron transportation/injection layer.
The above described light emitting device could be used as a display device. That display device could be applied to an electronic instrument. Alternatively, the above described light emitting device could be applied to an electronic instrument, as described below.
The description now turns to some examples of the materials that can be used in the various parts of the light emitting device in accordance with the present invention. These materials are merely given as a sample of known materials, but of course it is equally possible that various other materials could be selected.
Light-Emitting Layer
The material of the light-emitting layer can be selected from chemical compounds that are known for producing light of a predetermined wavelength. Organic light-emitting materials are preferable as the material of the light-emitting layer, for reasons such as the wide variety thereof and their capability of forming films.
Examples of such organic light-emitting material include aromatic diamine derivatives (TPD), oxydiazole derivatives (PBD), oxydiazoledimer (OXD-8), distyrylarylene derivatives (DSA), beryllium benzoquinolinol complexes (Bebq), triphenylamino derivatives (MTDATA), rubrene, quinacridone, triazole derivatives, polyphenylene, polyalkylfluorene, polyalkylthiophene, azomethine-zinc complexes, porphylin-zinc complexes, benzoxazole-zinc complexes, and phenanthroline-europium complexes, as disclosed by way of example in Japanese Patent Application Laid-Open No. 10-153967.
Various known materials can be used as the material of the light-emitting layer, such as those disclosed in Japanese Patent Application Laid-Open Nos. 63-70257, 63-175860, 2-135361, 2-135359, 3-152184, 8-248276, and 10-153967. These chemical compounds could not only be used alone, two or more of them could be mixed for use.
Hole Transportation/Injection Layer
If a light-emitting layer formed of an organic chemical compound is used in the light-emitting element section, a hole transportation/injection layer could be provided between the anode and the light-emitting layer, if necessary. A hole injection material of a known optically transmissive material could be used as the material of the hole transportation/injection layer, or it could be selected from among known materials used as hole injection layers in organic light emitting devices. The material of the hole transportation/injection layer has the function of either injecting holes or acting as a barrier to electrons, and it can be either an organic or a non-organic material. Specific examples are disclosed in Japanese Patent Application Laid-Open No. 8-248276, by way of example.
Electron Transportation/Injection Layer
If a light-emitting layer formed of an organic chemical compound is used in the light-emitting element section, an electron transportation/injection layer could be provided between the cathode and the light-emitting layer, if necessary. The material of the electron transportation/injection layer could have the function of transferring electrons that have been injected from the cathode into the light-emitting layer, and this material could be selected from known materials. A specific example that could be cited is disclosed in Japanese Patent Application Laid-Open No. 8-248276.
Each layer included in the light emitting device can be formed by known methods. For example, a suitable film formation method could be selected for the layers of the light emitting device, depending on the material properties thereof, such as vapor deposition, spin-coating, Langmuir-Blodgett method, or an ink-jet method.